Experiment and numerical simulation of bubbly two-phase flow across horizontal and inclined rod bundles

Experimental and numerical analyses were carried out on vertically upward air-water bubbly two-phase flow behavior in both horizontal and inclined rod bundles with either in-line or staggered array. The inclination angle of the rod bundle varied from 0 to 60° with respect to the horizontal. The measured phase distributions indicated non-uniform characteristics, particularly in the direction of the rod axis when the rods were inclined. The mechanisms for this non-uniform phase distribution is supposed to be due to: (1) Bubble segregation phenomenon which depends on the bubble size and shape; (2) bubble entrainment by the large scale secondary flow induced by the pressure gradient in the horizontal direction which crosses the rod bundle; (3) effects of bubble entrapment by vortices generated in the wake behind the rods which travel upward along the rod axis; and (4) effect of bubble entrainment by local flows sliding up along the front surface of the rods. The liquid velocity and turbulence distributions were also measured and discussed. In these speculations, the mechanisms for bubble bouncing at the curved rod surface and turbulence production induced by a bubble were discussed, based on visual observations. Finally, the bubble behaviors in vertically upward bubbly two-phase flow across horizontal rod bundle were analyzed based on a particle tracking method (one-way coupling). The predicted bubble trajectories clearly indicated the bubble entrapment by vortices in the wake region.     

The turbulent mixing rate and the fluctuations of static pressure difference between adjacent subchannels in a two-phase subchannel flow

Turbulent mixing rate between adjacent subchannels in a two-phase flow has been known to be strongly dependent on the flow pattern. In this study, flow visualization was made to investigate the mechanism of the turbulent mixing between subchannels in a two-phase flow under hydrodynamic equilibrium conditions. The test channel was a vertical multiple channel consisting of two identical rectangular subchannels, and the working fluids were air and water. It was observed in slug-churn flows that a large scale inter-subchannel liquid flow occurs in front of the nose of a large gas bubble and behind the tail when the bubble axially passes through the subchannel, and thus a high turbulent mixing rate of the liquid phase results. In order to know driving force of such a large scale inter-subchannel flow, measurement of instantaneous static pressure difference between the subchannels was also made. The result showed that there is a close relationship between the liquid phase turbulent mixing rate and the magnitude of the pressure difference fluctuations.     

Eurofer corrosion by the flow of the eutectic alloy Pb-Li in the presence of a strong magnetic field

A new investigation of the Eurofer 97 corrosion by the MHD flow of the liquid eutectic alloy Pb-17Li is presented. The experimental data previously obtained in Riga are confirmed and an attempt to model this phenomenon is presented. The model is based on a thermodynamic analysis of the dissolution and electro-dissolution mechanisms, leading to a relevant boundary condition at the liquid-solid interface. Then, analyzing the MHD flow, guiding ideas and scaling laws are derived for the dissolution rate of the Hartmann wall. The results obtained in the regime, where the solid wall is assumed to remain planar, allow determining a plausible value for an important non-dimensional number, the dissolution number Di. A linear analysis leads to predictions on the mechanism responsible for the formation of streaks imbedded within the Hartmann layer and associated with the wall shape disturbance, as well as for the selection of the unstable mode. It is found that this mechanism is related to an additional contribution due to the electric current, based on an electro-dissolution number Ed.     

Non-linear dynamics of a two phase flow system in an evaporator: The effects of (i) a time varying pressure drop (ii) an axially varying heat flux

In this paper we study the phenomena of density wave oscillations (DWO) in a vertical heated channel. The homogeneous equilibrium model is used to simulate the flow in the two-phase region. The equations are solved numerically using a ‘shooting-method' technique. This in its turn employs an implicit backward finite difference scheme. The scheme can incorporate the movement of the interface. It is very elegant and does not involve storage of variables in large N×N matrices. This scheme is sufficiently general and can be used to simulate the dynamic behaviour when: (i) the heat flux imposed at the surface is non-constant, i.e. exhibits an axial variation; and (ii) the imposed pressure drop is varied periodically at a fixed frequency. A possible explanation for the conflicting reports of the effect of a periodic variation in heat flux is provided using a linear stability analysis and the D-partition method. The interaction of the natural frequency of the DWO and the fixed forcing frequency of the imposed pressure drop gives rise to various phenomena viz relaxation oscillations, sub-harmonic oscillations, quasi-periodic and chaotic solutions. To aid the experimentalist describe this infinite-dimensional system on the basis of his experimental results we discuss the characterisation using only the velocity time series data. This is done employing the method of delay coordinate embedding. The phase portraits, stroboscopic map and correlation dimension of the actual attractor are compared with that of the reconstructed attractor from the velocity time series.